1. Field of the Invention
The present invention relates to a drive device for an electric vehicle.
2. Description of Related Art
In recent years, electric vehicles that are driven by a motor or a motor generator have been widely used. When a rotor of the motor or the motor generator rotates, vibration and noise at a particular frequency is caused in the entire stator in response to a magnetic reaction force, with fixed points of the stator serving as nodes and intermediate portions between the fixed points serving as anti-nodes. That is, the stator may be deformed to cause noise and vibration when the electric vehicle is driven. In order to suppress such vibration and noise, Japanese Patent Application Publication No. 2005-304213 (JP-2005-304213-A) discloses pressing the entire outer peripheral surface of a stator core using an end plate, pressing intermediate portions between fixed portions of the stator core using a pressing plate, and disposing the fixed portions of the stator core at unequal intervals in the circumferential direction. JP-2005-304213-A thus proposes suppressing vibration of the stator core at a particular frequency.
In a drive device for an electric vehicle that incorporates a motor, rotation of a rotor causes cogging torque, which causes vibration and noise. Thus, it is proposed, in a drive device for an electric vehicle in which two motors are mounted in one casing, to suppress vibration and noise during rotation of the rotor by shifting the respective assembly positions of the two rotors in the rotational direction by a set angle to shift field excitation timings (see Japanese Patent Application Publication No. 06-284665 (JP-06-284665-A), for example).
Vibration and noise may be caused by resonance between vibration due to regenerative torque produced when regenerative braking is applied in an electric vehicle and structural vibration of the vehicle. In order to reduce noise during application of regenerative braking, it is proposed to make a comparison between the vibration frequency of a motor and the vibration frequency of a drive range to reduce regenerative torque in a drive range in which the vibration frequency of the motor and the structural vibration frequency of the vehicle resonate with each other. On the other hand, it is also proposed to increase regenerative torque in a drive range in which the vibration frequency of the motor and the structural vibration frequency of the vehicle do not resonate with each other. It is proposed to reduce vibration and noise caused by regenerative torque by reducing and increasing regenerative torque (see Japanese Patent Application Publication No. 2010-114978 (JP-2010-114978-A), for example).
In an electric vehicle drive device that includes a plurality of, for example two, motor generators, stator core plates with the same size are occasionally used for the stators. In this case, the two stators have the same specific frequency. Therefore, the two stators may resonate with each other to cause significant vibration and noise.